Ascospores of Neurospora crassa, and those of various other heterothallic and homothallic Neurospora species, are normally ellipsoid, similar to the American football. In 1966, Mary Mitchell isolated a mutant with ascospores that are round rather than ellipsoid. A similar Round-spore mutant was reported in N. tetrasperma (Novak and Srb 1973). The dominant N. crassaRound-spore mutant (R) is altered in three ways: abnormal vegetative morphology, female sterility, and ascospore shape. The round-ascospore phenotype is ascus-dominant, meaning that in crosses of wild type x R, all eight ascospores are phenotypically round, R as well as R+ (Figs 1-3; Mitchell 1966; Raju 1992; Perkins et al. 2001).
In wild type x R, early ascus development and meiosis are normal (Fig 4). Postmeiotic mitosis occurs but the spindles at this division are aligned abnormally (Figs 5, 6). Following the third division, all eight nuclei line up in single file and cut out eight round ascospores (Figs 7-10). Another mitosis in the young ascospores makes them binucleate (Figs 11-15). During maturation, ~90% of round ascospores differentiate a single germ pore, rather than the normal two in the ellipsoid wild-type ascospores (Figs 16-17). In contrast, the round ascospores of N. tetrasperma mutant form two germ pores.
We have observed another abnormality during ascospore cutting: In up to 5% of asci, one or two ascospores show bud-like vesicles (Figs 18-22). In the developing asci, ascospore-delimiting membranes are formed at the periphery of the ascus during interphase II. Following a postmeiotic mitosis, these membranes invaginate to sequester individual nuclei and the immediately surrounding cytoplasm into eight ascospores. Since eight round ascospores require less spore-delimiting membrane than do eight ellipsoid ascospores, there appears to be a surplus. We interpret the ascospore buds in the round-spored asci as being the unused wall membranes. Inclusions in the giant ascospores of the Banana mutant can also be interpreted as unused remnants of wall forming membranes (Raju and Newmeyer 1977).
The Round-spore mutant has been extensively used in studies of suppression of dominant round spore phenotype by Sad-1 and Sad-2 mutants (Figs 23, 24; Shiu and Metzenberg 2002; Shiu et al. 2001).
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